Aluminum coating of ferrous metal articles



United States Patent ALUMINUM COATING 0F FERROUS METAL ARTICLES Kenneth James Brondyke, Oakmont, Pa., assiguor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania N0 Drawing. Application January 2, 1953, Serial No. 329,469

Claims. (Cl. 117-51) This invention relates to coating ferrous metal articles with aluminous metal, i. e. aluminum and aluminum base alloys, by the dip process. The invention finds particular utility in continuously applying an aluminous metal coating to such products as wire or strip.

Aluminum coated wire or steel articles have been long known for their resistance to corrosion and oxidation at elevated temperatures as Well as their pleasing appearance. The aluminum coatings have been applied in various ways such as by dipping in the molten metal, spraying, or heating the ferrous article in contact with aluminum powder.

, While these methods have been satisfactory for the batch treatment of single articles or groups of articles, they have not been satisfactory for the continuous treatment of greatly elongated articles such as Wire or strip. One of the drawbacks in continuously treating such articles has been the inability to produce a uniform and firmly adherent coating of aluminum, particularly where the article is in contact with the coating metal for only a very short time. This is especially true where Wire or strip is passed through a molten metal bath at a relatively high speed. These and other difficulties have led me to devise a method which makes it possible to quickly and effectively apply an aluminous metal coating to ferrous articles, especially in the continuous treatment of wire or strip.

One of the objects of my invention is to provide a method for quickly and uniformly applying a flux to a ferrous article at a relatively low temperature which acts quickly to promote adhesion of aluminous metal to the ferrous surface. Another object is to provide a method of fluxing a ferrous surface which does not yield objectionable fumes. Still another object is to provide a method of continuously applying a strongly adherent aluminous coating of uniform thickness to moving elongated ferrous articles. A further object is to provide a smooth strongly adherent coating of aluminous metal upon moving steel wire or strip as it passes through a molten aluminous metal bath. Another object is to provide a flux which is readily soluble in water, contains no chlorides, and effectively and quickly prepares a ferrous surface for reception of an aluminous coating.

My invention is based on the discovery that a uniform tight aluminous metal coating can be formed on ferrous articles, especially moving wire or strip, by covering the article with a dry special salt flux and then immersing the flux coated article in a molten aluminous bath for a short period of time. I have formulated a chloride-free flux which is very effective in cleaning a ferrous metal surface and can be applied to the ferrous body in the form of an aqueous solution and therefore is particularly suited to use in a continuous aluminous metal coating operation. By employing a solution instead of a fused salt bath to supply a flux coating the problems associated with heating and maintaining a molten salt bath are, of course, obviated. Moreover, through use of an aqueous solution the thickness of the flux coating can be controlled and only enough flux need be applied to clean the ferrous surface thus avoiding excessive drag-out of flux such as often occurs where a fused salt bath is used. In addition, the flux contains no constituents which give rise to a fume problem such as encountered where ammonium chloride or zinc chloride is employed.

The special flux which is adapted to use in my metal coating process contains at least one alkali metal bromide and a potassium fluoride as the principal and essential components in such proportions as to yield a mixture having a melting point below that of the fused aluminous metal being applied to the ferrous metal body. The flux is readily soluble in water, adheres well to the ferrous surface in the dry condition and provides the necessary fluxing action to permit quick and effective contact be tween the molten aluminous metal and the ferrous article. A mixture consisting essentially of 65 to by weight of the akali metal bromide and from 15 to 35% of at least one fluoride of the group composed of potassium fluoride and potassium acid fluoride will have the desired low melting point range. It is important that the flux should fuse as it comes in contact with the molten aluminous metal in order to obtain the desired removal of oxide film on the ferrous surface and to establish the necessary wetting of that surface by the fused metal. In my preferred practice the melting point of the flux should not exceed 1200 F.

The potassium fluoride component of the flux, as indicated above, may consist of re normal fluoride (KP), the acid fluoride (KRHF), or both. Where the acid fluoride is employed no difliculty arises from occurrence of objectionable fumes if conventional ventilation practices are followed which are used in dip coating processes. Inasmuch as the potassium fluoride portion of the acid fluoride supplies the stable flux component, allowance must be made for the hydrofluoric acid portion of the salt when calculating the amount of acid fluoride required to produce a mixture having a predetermined melting point.

0f the alkali metal bromides those of potassium, sodium and lithium are preferred because of their commercial availability. Also, two or more of the alkali metal bromides may be used simultaneously. The combination of potassium bromide and potassium fluoride or acid fluoride has given particularly satisfactory results. In such a flux the ratio of bromide to the metal fluoride should be about three to one to obtain the lowest melting point mixture.

It is to be understood that the flux may contain the usual impurities associated with bromides and fluorides in their commercial forms. The amount of such impurities is generally very small and of no importance as far as affecting the action of the flux is concerned. Under some conditions it may be desirabe to add small amounts of water soluble fluorides of the heavy metals, such as titanium tetrafluoride, to improve certain. properties of the flux. The total amount of such fluorides in any case should not exceed about 10% by Weight of the flux. In making such additions care must be exercised to avoid raising the melting point of the flux above that of the aluminous metal bath.

In order to obtain freedom from the corrosive effect of chlorides none are permitted in the mixture other than ferrous article in the form of an aqueous solution rather; than by immersion in a fused salt bath. It has been found that by passing the ferrous article through the flux solo-i tion and subsequently drying the article a suflicient amount of flux is uniformly deposited on the article to remove any oxide film and to establish a good contact between the ferrous and aluminous metals during the metal coat-- ing operation. By applying the flux in this manner a smaller quantity of flux is used than where the' article is immersed in a fused salt bath and the amount of flux deposited can be closely controlled by controlling the concentration of salt in'the solution. From the standpoint of large scale operations such close control is of considerable importance for it eliminates or greatly minimizes loss and waste of flux. The quantity of fiux which should be added to water to'provide the proper concentration is in the proportion of 0.01 to 0.25 pound per pound of water which on a percentage basis amounts to about 1 to Although the maximum quantity is well below the limit of solubility of the salts in water there is nothing to be gained from using larger proportions of the flux.

While the solution can be used at room temperature, the desired flux coating can be produced more quickly if the solution is heated to a temperature between 170 and 190 F. In the latter case it is desirable to hold the article in the solution long enough for it to attain the temperature of the solution so that upon emerging therefrom the water will quickly evaporate and leave a dry fiux deposit and thus obviate any need for a special drying operation. The period of immersion in the hot aqueous solution will, of course, depend upon the size of the ferrous article and the area to be flux coated, but ordinarily the period of immersion will be less than one minute. In treating wire or strip the article quickly reaches the temperature of the solution which means that wire or strip can be passed'through the solution at a relatively rapid rate. Where the solution is at room temperature or slightly higher, a drying operation will be required to evaporate the water, especially in continuous operations. In any case the flux should be dry when the article is immersed in the molten aluminous metal.

The aluminous metal into which the flux coated ferrous metal article is dipped may consist of aluminum or an aluminum base alloy. In the case of the alloys, a composition should be selected which will afford the desired protection to the underlying ferrous metal. For example, if the coated article is to be exposed to a corrosive environment, the aluminous metal should have an electrode potential sufficiently above that of the ferrous metal to provide electrolytic protection of any exposed areas and consequently those metal alloy additions which tend to reduce the electrode potential of aluminum, such as copper and nickel, should be avoided. On the other hand, such alloying elements as silicon and zinc are advantageous additions because they are either neutral or beneficial with respect to raising the electrode potential of aluminum. Aluminum-silicon alloys containing from 2 to 13% silicon may be employed or aluminum-zinc alloys containing from 0.1 to 5% zinc are satisfactory. Other known elements having similareffects may be used with silicon or zinc or in place of them. The small amounts of impurities commonly found in aluminum and the aluminum base alloys have no significant effect upon the performance of the aluminous coating.

To ensure adequate wetting of the ferrous metal article and a satisfactory aluminous metal coating, the temperature of the metal bath should be maintained within the range of 1250 to 1400 F. Below 1250 F. there is danger of too rapid a-chilling of the coated ferrous article with consequent danger of a non-uniform coating while on the other hand, if a temperature of 1400" F. is exceeded, an excessive amount of a brittle aluminum-iron constituent will be formed at the interface of the ferrous and aluminous metals with resultant loss in adhesion of the coating to the base metal. The ferrous metal article should be immersed inthe molten bath for a long enough period of time to provide the desired thickness of coating; a greater thickness may be obtained as the period of immersion increases.

Generally, an immersion period at bath temperature of less than half a minuteis adequate. In the case of continuous treatment of wire or strip the speed with which it is passed through the molten bath must be adjusted to permit sufficient immersion or the temperature and size of the bath must be altered to provide the proper length of immersion period. Ordinarily, steel wire may be passed through the bath at the. rate of 10 to feet per minute and the wire should be immersed in the bath for from 2 to 10 seconds.

T he thickness of the aluminous metal coating can be varied as desired, as indicated above, the thickness required in any given case being determined by the nature of the service to which the coated article is subjected. Usually a coating thickness of 0.0002 to 0.0015 inch will provide the necessary protection.

Any of the common types of iron and steel may be coated with the aluminum metal in the manner described above. Generally mild steels, cooper bearing steels and low ailoy high strength steels are the ones employed where an aluminous coated product is desired. All of'these iron base alloys as well as iron itself are referred'to herein as being ferrous metals.

As an illustration of coating steel wire, the following example may be cited. A mild steel in the form of'0.l09 diameter wire was passed through an aqueous solution containing 0.10 pound of a salt mixture per pound. of

water composed of 75% by weight of potassium bromide and 25% potassium fluoride. The solution was maintained at a temperature of to F. and the wire passed through that solution at a rate of 30 feet per minute; Upon emergence from the aqueous solution the wire dried and it was immediately immersed in a molten aluminum bath maintained at a temperature of 1300 F. At the rate of travel of wire the period ofimmersion lasted 5 seconds. Upon emergence from the metal baththe wire was uniformly coated with aluminum, the coat ing being 0.0005 inch in thickness.

Having thus described my invention and at a preferred embodiment thereof, I claim:

1. The method of coating a ferrous metal body with aluminous metal by the dip coating process wherein the aluminous metal is directly bonded to the ferrous metal body comprising covering the surface of said ferrous metal body with a dry solid chloride-free flux, said flux containing as its essential components from 65 to 85% by weight of at least one alkali metal bromide andfrom' 15 to 35% of at least one fluoride of the group consisting of potassium fluoride and potassium acid fluoride, immersing said flux covered body in a bath of molten aluminous metal and thereafter withdrawing the body from the molten bath.

2. The method of coating a ferrous metal body with aluminous metal by the dip coating process wherein the aluminous metal is directly bonded to the ferrous'metal body comprising covering the surface of said ferrous metal body with a dry solid chloride-free flux, said. flux containing as its essential components from 65 to 85% by weight of at least one bromide of the group consisting of sodium bromide, potassium bromide and lithium bromide and from 15 to 35% of at least one'fluoride of the group consisting of potassium fluoride and potassium acid fluoride, immersing said flux covered body in a bath of molten aluminous metal and thereafter withdrawing the 'body from the molten bath.

3. The method of coating a ferrous metal body with aluminous metal by the dip coating process wherein the aluminous metal is directly bonded to the ferrous metal body comprising forming an aqueous solution of a chloride-free flux containing as its essential components from 65 to 85%' by weight of at least one alkali metal bro-' mide and from 15 to 35% of at least one fluoride of the group consisting of potassium fluoride andpotassinm acid fluoride, the proportion of flux to water. being,0.0l' to 0.25 lb. of flux to 1 lb. of water, immersing said ferrous metal'body in said solution, withdrawing it therespawns from, drying the body thereby leaving a dry flux deposit on surface of said body, immersing said flux covered body in a molten aluminous metal bath and thereafter with drawing said body from said bath.

4. The method of coating a ferrous metal body with aluminous metal by the dip coating process wherein the aluminous metal is directly bonded to the ferrous metal body comprising forming an aqueous solution of a chloride-free flux containing as its essential components from 65 to 85 by weight of at least one alkali metal bromide and from 15 to 35% of at least one fluoride of the group consisting of potassium fluoride and potassium acid fluoride, the proportion of flux to water being 0.01 to 0.25 lb. of flux to 1 lb. of water, heating said solution and maintaining it within the temperature range of 170 to 190 F., immersing said ferrous metal body in said hot solution for a sufficient length of time to bring the body up to a temperature within said range, withdrawing said body from said solution, allowing the water to evaporate and leave a dry flux deposit on the surface of said body, immersing said flux covered body in a molten aluminous metai bath and thereafter withdrawing said body from said bath.

5. The method of coating a ferrous metal body with aluminous metal by the dip coating process wherein the aluminous metal is directly bonded to the ferrous metal body comprising forming an aqueous solution of a chloride-free flux containing as its essential components from 65 to 85% by Weight of at least one alkali metal bromide and from 15 to 35% of at least one fluoride of the group consisting of potassium fluoride and potassium acid fluoride, the proportion of flux to water being 0.0! to 0.25 lb. of flux to 1 lb. of Water, immersing said ferrous metal body in said solution, withdrawing it therefrom, drying the body thereby leaving a dry flux deposit on the surface of said body, immersing said flux covered body in a molten aluminous metal bath maintained at a temperature within the range of 1250 to 1400 F. for a period at bath temperature of not over /2 minute and withdrawing said body from the molten bath.

6. The method of coating a ferrous metal body with aluminous metal by the dip coating process wherein the aluminous metal is directly bonded to the ferrous metal body comprising forming an aqueous solution of a chloride-free flux containing as its essential components from 65 to 85% by weight of at least one bromide of the group consisting of sodium bromide, potassium bromide and lithium bromide, and from 15 to 35% of at least one fluoride of the group consisting of potassium fluoride and potassium acid fluoride, the proportion of flux to water being 0.01 to 0.25 lb. of flux to 1 lb. of water, heating said solution and maintaining it within the temperature range of 170 to 190 F., immersing said ferrous metal body in said hot solution for a suflicient length of time to bring the body up to the temperature of the solution, withdrawing said body from said solution, allowing the water to evaporate and leave a dry flux deposit on the surface of said body, immersing said flux covered body in a molten aluminous metal bath maintained at a temperature Within the range of 1250 to 1400 F. for a period at bath temperature of not over A minute and withdrawing said body from the molten bath.

7. The method of coating a ferrous metal body with aluminous metal by the dip coating process wherein the aluminuous metal is directly bonded to the ferrous metal body comprising forming an aqueous solution of a chlo ride-free flux composed of from to by weight of at least one alkali metal bromide, from 15 to 35% of at least one fluoride of the group consisting of potassium fluoride and potassium acid fluoride, and up to 10% of a heavy metal water soluble fluoride, the proportion of flux to water being 0.01 to 0.25 lb. of flux to 1 lb. of water, immersing said ferrous metal body in said solution for a period of less than 1 minute, withdrawing said body from said solution, drying the body thereby leaving a dry flux deposit on the surface of said body, immersing said flux covered body in a molten aluminous metal bath and thereafter withdrawing said body from the molten metal bath.

8. The method of continuously coating ferrous metal wire or strip with aluminous metal by the dip coating process wherein the aluminous metal is directly bonded to the ferrous wire or strip comprising forming an aqueous solution of a chloride-free flux containing as its essential components from 65 to 85% by weight of at least one alkali metal bromide and from 15 to 35% of at least one fluoride of the group consisting of potassium fluoride and potassium acid fluoride, heating said solution and maintaining it within the temperature range of to F., passing said wire or strip through said hot solution, the length of the immersion period being suflicient to bring the wire or strip up to the temperature of the solution, withdrawing said wire or strip from the solution, allowing the water to evaporate therefrom, immersing said flux covered wire or strip in a molten aluminous metal bath maintained at a temperature within the range of 1250 to 1400 F. for a period of not over /2 minute and withdrawing said wire or strip from the molten metal bath.

9. A chloride-free salt flux adapted to clean the surface of a ferrous metal body preparatory to receiving an aluminous metal coating, said flux containing as its essential components from 65 to 85% by weight of at least one alkali metal bromide, from 15 to 35% of at least one fluoride of the group consisting of potassium fluoride and potassium acid fluoride, and up to 10% of water soluble heavy metal fluoride.

10. A salt flux adapted to clean the surface of a ferrous metal body preparatory to receiving an aluminous metal coating, said flux consisting of from 65 to 85 of potassium bromide and from 15 to 35 of at least one fluoride of the group consisting of potassium fluoride and potassium acid fluoride, said flux having a melting point below 1200 F.

References Cited in the file of this patent UNITED STATES PATENTS 527,478 Broadwell Oct. 16, 1894 648,831 Bates May 1, 1900 FOREIGN PATENTS 313,487 Great Britain July 25, 1930 

1. THE METHOD OF COATING A FERROUS METAL BODY WITH ALUMINOUS METAL BY THE TOP COATING PROCESS WHEREIN THE ALUMINOUS METAL IS DIRECTLY BONDED TO THE FERROUS METAL BODY COMPRISING CONVERING THE SURFACE OF SAID FERROUS METAL BODY WITH A DRY SOLID CHLORIDE-FREE FLUX, SAID FLUX CONTAINING AS IT ESSENTIAL COMPONENTS FROM 65 TTO 85% BY WEIGHT OF AT LEAST ONE ALKALI METAL BROMIDE AND FROM 15 TO 35% OF AT LEAST ONE FLUORIDE OF THE GROUP CONSISTING OF POTASSIUM FLUORIDE AND POTASSIUM ACID FLUORIDE, IMMERSING SAID FLUX COVERED BODY IN A BATH OF MOLTEN ALUMINOUS METAL AND THEREAFTER WITHDRAWING THE BODY FROM THE MOLTEN BATH. 